moving a mailserver

My company is moving offices this weekend. We have one email server at the
old office. I'd like to move it this Friday night. It would be cool if we
didn't lose any email.
Our DNS holds an MX record for our domain with a TTL of 1800 seconds (30
mins) and an MX level of 10
Choice 1:
1-change the MX record to point to the new IP address which the mail server
will have in the new building
2-unplug the mail server and drive it to the new building (10 mins away),
assign the box the new ip address
Is anyone likely to ignore the 1800 TTL and continue sending email to the
old ip address for longer than 30 mins? during the downtime, will sending
email attempts fail or simply keep trying?
Choice 2:
1-set up a new mail server in the new building
2- add an MX record with an MX record of 20
3- set up a store and forward config to forward the email to the original
email server
the idea being that someone sending email will first try the email with the
10 MX level, and failing that will try the 20 MX level server. that server
will accept the email and attempt to forward it to the original email
server. it should keep trying for the 10 minutes the original email server
is down and then succeed once it's back up at the new place.
who likes choice 1? choice 2? other, more intelligent, choices?
Ryan